Abstract - Branch-lines between normal faults and their sub-parallel splays mapped from 3D seismic reflection data show a range of forms from straight lines to closed loops. The different geometrys are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip-lines with fault branch-lines. The geometries of these normal fault branch-lines are similar to those for thrusts previously inferred from limited 2D data. The orientations of the axes of relays and associated bends relative to a fault slip direction is identified as an important control on the structures developed within relay zones. Neutral, restraining and releasing bends can each occur with all three fault modes (normal, reverse and strike slip: see figure), but data bias is a major factor in determining which bend type is most often observed within each fault mode.
On normal faults the initial relay zone geometry controls the dominant branch-line orientation and the same control is likely on branch-lines associated with the other modes of faulting. A review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of the fault surface and slip directions. This relative orientation determines how the relay strain is accommodated and hence the degree of hard-linkage and development of branch-lines.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Tectonic Studies Group, University of St Andrews, December 1999